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New fragment header, was: Re: Evolution of the IP model - ICMP and MTUs
Ok, this is what the new fragment header would look like:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| next header |res|type |S/F|T| fragment offset |res|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source port | destination port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| observed packet length | checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
next header: protocol number of the header following the new fragment
header
res: reserved
S/F: SYN/FIN, copied/synthesized from transport
T: 1 = addresses were translated
fragment offset: as usual
type: tcp/udp/sctp/dccp/AH/ESP/other+ports/other-ports
M: more fragments
identification: groups fragments of the same packet, copied from TCP
sequence if applicable
ports: copied from transport port numbers or IPsec SA
observed packet length: maximum packet size seen in the opposite
direction in the last 5 minutes, 0 = unknown / no packets larger than
1280 bytes seen
checksum: checksum of the IP pseudo header + transport segment bytes
upto byte 1280 of the packet, 0 = N/A
The new fragment header is always the first header following the IPv4
or IPv6 header. This means that filtering on protocol and port numbers
is always easy because the information is always present in the same
place in the packet, even if additional headers are present between
the new fragment header and the transport header.
When routers observe a packet with the new fragment header that is too
large to be forwarded, they truncate the packet. The minimum size of
the first fragment is 1280 bytes. Note that traditional IPv4
fragmentation produces the same result as truncation for the first
fragment.
Because the new fragment header has its own checksum over the relevant
part of the first 1280 bytes of the packet, bit errors can still be
detected so protocols that can make use of the truncated data (such as
TCP or DNS) may do so.
NATs MAY only translate the new fragment header.
The observed packet length field allows for passive PMTUD. A full
PMTUD implementation can be done by initializing the PMTU to 1280 and
then use a new fragment header for packets larger than the PMTU and/or
in response to incoming packets that updated the OPL, updating the
PMTU from the OPL field in incoming packets, resetting the PMTU after
a retransmission timeout.